Glossary: K through O

Kepler's Laws
Three fundamental laws of planetary motion first observed by Johannes Kepler in the late sixteenth century.

The laws state:
• First Law: The orbit of a planet is an ellipse with the Sun at one focus.
• Second Law: The radius vector sweeps over equal areas in equal time intervals.
• Third Law: The period of a planet squared is proportional to the orbital distance cubed. What this law means is that the further you are from the Sun, the longer it takes to orbit it.

Light Year (lyr)
The distance that light travels in one year, equal to 10,000,000,000,000 km, or 63,000 AU.

MACHOs
MACHOs are objects about the size of Jupiter. Macho is short for MAssive Compact Halo Object. Machos are the only type of dark matter for which there is direct evidence.

Check out these CfPA MACHO links:
• The MACHO experiment
• Big bang cosmology primer

Mars
The fourth planet from the Sun, Mars is a terrestrial planet. Mars is smaller than Earth, and is sometimes called the red planet. Check out SEDS' Nine Planets, for more about Mars. Or see StarDate's Solar System Guide.

Mercury
With an orbit of only 58 million km, Mercury is the closest planet to the Sun. It is a terrestrial planet, and has a diameter of about 5000 km. Check out SEDS' Nine Planets, for more about Mercury. Or see StarDate's Solar System Guide.

Meridian
An imaginary line connecting the north and south celestial poles and passing through the zenith. See celestial sphere for a helpful diagram.

Meteor
The trail of light seen in the sky when a meteorite burns up in Earth's atmosphere. Also called shooting stars.

Meteorite
Interplanetary material which is pulled into Earth's atmosphere by gravity, and burned up by friction with the atmosphere.

Milky Way Galaxy
Our home galaxy, the Milky Way is thought to be an average spiral galaxy.

Moon
With a diameter of 3500 km, Earth's Moon is actually quite large for a moon. It orbits the Earth at a distance of 385,000 km. Since the same side of the moon always faces Earth, a Moon 'day' is 29.5 Earth days. It is thought that when the Earth formed it had no moon. Then, about 4.5 billion years ago a small planet hit Earth. The impact caused the Earth's surface to become molten, and the small planet became gravitationally bound to Earth, becoming the Moon. Although it can be confusing, the word moon is also used generically to mean any object which orbits another object. Check out SEDS' Nine Planets, for more about the Moon. Or see StarDate's Solar System Guide.

Nebula
A cloud of interstellar gas. One of the most famous nebula is the Great Orion Nebula, which can be seen with the unaided eye. It is the middle 'star' of the sword of Orion. For excellent nebulae pictures, check out HST Images.

Neptune
The eighth major planet, Neptune is a gas giant planet, which orbits the Sun every 165 years.
Check out SEDS' Nine Planets , for more about Neptune. Or see StarDate's Solar System Guide.

Neutrino
A class of elementary particle thought to have either no mass or very little mass. If neutrinos have any mass, then they may account for some fraction of dark matter. For more on neutrinos, check out this big bang cosmology primer.

Neutron
A sub-atomic particle found in the nuclei of an atom. Unlike the proton and electron, the neutron has no net charge.

Nuclei
The core of an atom, the nuclei consists of various numbers of protons and neutrons, depending upon the element.

Omega
The ratio of the observed density of the Universe to the critical density of the Universe is given by the Greek letter omega. If omega is less than one the Universe will continue expanding until it is so large that it dies a cold death. If omega equals one the Universe will eventually stop expanding but will not collapse; the Universe will die a cold static death. If omega is greater than one, then the Universe will die a hot, fiery death in a Big Crunch. For theoretical reasons, cosmologists believe that omega = 1.
For more about omega check out the big bang cosmology primer.